What is /bin/true?

On a Linux system, what is /bin/true? Wha开发者_JS百科t is it used for?

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/bin/true is a command that returns 0 (a truth value in the shell).

Its purpose is to use in places in a shell script where you would normally use a literal such as "true" in a programming language, but where the shell will only take a command to run.

/bin/false is the opposite that returns non-zero (a false value in the shell).

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From the man page:

true - do nothing, successfully

true returns a status 0.

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Note, it's not just silly or visually nice. It helps for example to exit a program without activating the end handlers which might mess up when doing multi threading or forked programs. Like in perl:

#!/usr/bin/env perl

exec "/bin/true";

END {
  print "This wont get printed .. would have if I just 'exit' or 'die'\n";
}

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I've seen it used to fool a system operation into thinking a command has run when it hasn't. If a command is faulty eg looping, you can replace it with a symlink to 'true' to get the master job to run. Only a good idea if the job replaced isn't essential.

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Simply saying its a program returning 0. Sometimes we need to get this value to let the script more readable. It is usually used when you need to use a command for a true value.

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In the UNIX shells, it's used for the same purposes as boolean constants true and false in any other language.

while true; do
    something
done

flag=true
...
if $flag; then
    something
done